Plastic containers are easy to adsorb and absorb odor and are inferior in gas barrier properties to bottles or cans, and therefore it has been difficult to use plastic containers for storing drinks sensitive to oxygen such as beer and sparkling liquor. In order to overcome drawbacks of plastic containers such as strong adsorption and absorption properties and low gas barrier property, a method and apparatus for coating a plastic container with, for example, a hard carbon film (diamond-like carbon (DLC)) and the like is disclosed. For example, an apparatus for coating the inner wall surface of a container with a hard carbon film is disclosed which uses an external electrode having an internal space whose shape is substantially similar to the outer shape of a container to be coated and an internal electrode adapted to be inserted into the inside of the container through the mouth of the container and also serving as a raw material gas introduction tube (see, for example, Patent Document 1 or 2). Such an apparatus is used by applying high-frequency power to the external electrode in a state that a carbon source gas such as carbons of an aliphatic hydrocarbons or an aromatic hydrocarbons is supplied into the container as a raw material gas. At this time, plasma is generated from the raw material gas between the external electrode and the internal electrode. Ions in the generated plasma are attracted by a high-frequency-derived potential difference (self-bias) generated between the external electrode and the internal electrode and impinge on the inner wall of the container so that a film is formed. Here, as a power source for plasma generation, one with an industrial frequency of 13.56 MHz which is easy to use and easily available is used in a conventional apparatus for mass production.
Meanwhile, a technique for suppressing the generation of plasma in an exhaust chamber of the apparatus or in an exhaust path downstream from the exhaust chamber to prevent the generation of carbon-based foreign matters is disclosed (see, for example, Patent Document 3). According to this technique, a spacer made of a dielectric material is provided in a gap between the inner wall surface of the external electrode and the outer wall surface of the plastic container to adjust the total capacitance of the apparatus, and low-frequency power of 400 kHz to 4 MHz is supplied to the external electrode. Another technique is disclosed (see, for example, Patent Document 4), according to which a vacuum chamber obtained by replacing the upper part of the external electrode with a dielectric material is used to adjust the total capacitance of the apparatus and low-frequency power of 400 kHz to 4 MHz is supplied to the external electrode.
Further, a technique for suppressing the formation of an excess thin film on the mouth of a container is disclosed (see, for example, Patent Document 5). According to this technique, the length of a discharge plasma sheath and the radius of the mouth of a container are set so that a predetermined relationship can be maintained between them and a low-frequency power source of 0.1 to 5 MHz is used.